The Trp operon is a coordinately regulated group of genes (trpA–trpE) that are required for tryptophan biosynthesis in E. coli. Based on the figure above, which of the following correctly describes the regulation of the Trp operon?

Question

The Trp operon is a coordinately regulated group of genes (trpA–trpE) that are required for tryptophan biosynthesis in E. coli. Based on the figure above, which of the following correctly describes the regulation of the Trp operon?

BlackTom AI · Accepted Answer

When evaluating regulation of the Trp operon, it's important to connect the presence or absence of tryptophan to whether the repressor is activated to block transcription.
Option 1: 'In the absence of tryptophan, the repressor is active and binds to the Trp operator, preventing RNA polymerase from transcribing the operon.' This is incorrect because the repressor requires tryptophan as a corepressor to become active. Without tryptophan, the repressor remains inactive and cannot bind the operator to block transcription.
Option 2: 'In the presence of tryptophan, the repressor is active and binds to the Trp operator, preventing RNA polymerase from transcribing the operon.' This statement aligns with the classic model: tryptophan acts as a corepressor that activates the Repressor (R) protein, enabling it to bind the operator and block RNA polymerase, thereby repressing transcription.
Option 3: 'In the absence of tryptophan, the trpR gene is inactive, preventing the production of the repressor that blocks expression of the operon.' This is inaccurate because the trpR gene is not regulated by tryptophan in the sense of turning production on or off; the repressor protein is produced, but it only becomes functionally active when tryptophan is present as a corepressor. Thus no repressor activity occurs in the absence of tryptophan.
Option 4: 'In the presence of tryptophan, the trpR gene is inactive, preventing the production of the repressor that blocks expression of the operon.' This is incorrect because the presence of tryptophan does not switch off trpR transcription; the issue is the activity state of the repressor protein, which is controlled by tryptophan binding, not by shutting down trpR expression entirely.

The Trp operon is a coordinately regulated group of genes (trpA–trpE) that are required for tryptophan biosynthesis in E. coli. Based on the figure above, which of the following correctly describes the regulation of the Trp operon?单项选择题

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The trp operon is regulated by three different mechanisms of regulation. These mechanisms include….. (select all that are correct)

When tryptophan binds to trp repressor, this causes trp repressor to __ to the trp operator and transcription.

When tryptophan concentrations are high in Escherichia coli, what happens?

When arginine binds to the repressor, the repressor does not bind to the operator.

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